Low pressure electric discharge lamp electrode



June 3, 1969 J. H. CAMPBELL ETAL 3,448,318

LOW PRESSURE ELECTRIC DISCHARGE LAMP ELECTRODE Filed Nov. 17, 1966 Their A=tbovne9 lnven tors. John H. CaTnpDeLL Detmav D. kershaw 8 United States Patent 3,443,318 LOW PRESSURE ELECTRIC DISCHARGE LAMP ELECTRODE John H. Campbell, Mentor, and Delmar D. Kershaw, Kirtland, Ohio, assiglors to General Electric Company, a corporation of New York Filed Nov. 17, 1966, Ser. No. 595,213 Int. Cl. H01j 17/06, 61/06, 61/08 US. Cl. 313-212 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to low pressure electric discharge lamps such as fluorescent lamps comprising a pair of thermionic electrodes sealed into opposite ends of an elongated tube containing mercury vapor and an inert gas for the ionizable medium. It is more particularly concerned with electrodes suitable for use in such lamps at relatively high currents and possibly in conjunction with an inert gas filling at a lower pressure or consisting at least in part of one of the lighter atomic weight inert gases.

The inert starting gas which has been most commonly used in the past is argon at a filling pressure between 2.5 and 3.5 millimeters of mercury. The starting gas is necessary to initiate the discharge between the electrodes and to protect the electrodes from destructive ion bombardment. The type of cathode commonly used comprises a compact or concentrated coil, for instance a coiled coil of tungsten wire provided with an overwind and carrying electron emitting materials such as alkaline earth metal oxides. These cathodes were first used in the older sizes of fluorescent lamps such as the common 40-watt lamp having an operating current of about 450 milliamperes. By increasing the diameter of the tungsten wires and the overall mass of the coil, they were adapted first to the 800 milliampere size of lamps, and finally to the 1500 milliampere fluorescent lamps including lamps of the kind utilizing configurated envelope of noncircular cross-section. Such cathodes perform quite satisfactorily provided the conventional argon starting gas filling is used at a pressure of 2.5 to 35 millimeters. However, in some lamp designs it is desirable to utilize a lower pressure of argon in order to reduce the starting voltage or increase the luminous efliciency, that is the ratio of lumens output to watt input. Alternatively, it may be desirable to use a lighter starting gas such as neon or a mixture of neon and argon in order to increase the operating voltage and achieve higher loading or lamp wattage. In either case, it may be found that the conventional design of coiled overwound oxide-coated cathode is not adequate to withstand over an acceptable life span the increased ion bombardment resulting from the higher cathode fall under these conditions.

The principal object of the invention is to provide improved cathode constructions suitable for operation at high currents, or in conjunction with starting gas filling pressures less than conventional, or in starting gas mixtures containing an appreciable proportion of inert gas of lower atomic weight than argon.

A more specific object of the invention is to provide cathode constructions capable of commercially acceptable lives at currents of 1500 milliamperes or more, for

example in argon at less than 2.5 millimeters pressure or in a starting gas containing an appreciable proportion of neon.

When it is attempted to use conventional type cathodes in a 1500 milliampere lamp with neon as the starting gas at 2 millimeters pressure, only a few hundred hours life can be achieved. Such cathodes develop a hot spot during operation, even when heated continuously by means of preheat windings on the ballast transformer, and most of the electron emission originates from this localized high temperature spot. This condition entails a higher ion and electron density and results in a higher cathode fall than is the case where the electron emission is distributed over a larger cathode area. The higher cathode fall in this instance is readily perceived because it manifests itself as a visible neon glow and it is accompanied by destructive ion bombardment of the cathode. We have found that the hot spot can be eliminated by utilizing an indirectly heated electrode consisting of an oxide coated nickel sleeve with an internal heater. Such a cathode is essentially unipotential and electron emission occurs uniformly over the entire activated area with the result that the hot spot and the accompanying neon glow are eliminated. However, such a cathode is not feasible for an electric discharge lamp because in the dimensions required, an excessive amount of energy would be needed to heat it. This would means relatively low efliciency and also delay at starting in order to bring the cathode up to operating temperature.

In accordance with our invention, we have found that the ideal operating characteristics of the indirectly heated equipotential cathode may be achieved without its disadvantages by stretching out a coiled overwound type cathode to the point where the spacing between turns in the final coiling is comparable to the turn diameter, that is not much greater nor much less than the turn spacing.

By way of example, the turn spacing may be equal to the turn diameter and we may take several strands of such stretched filament and assemble them in a bridge configuration wherein they are connected electrically in parallel or in a series parallel combination. In general, a cathode should comprise at least 4 centimeters of such stretched filament per ampere of lamp discharge current to be supplied by it. Such electrodes, which may conveniently be referred to as bridge electrodes, present a large activated surface spread over a wide area and at the same time have the required resistance to operate within the voltage and current limits of conventional preheat windings. We have found that such cathodes operate without hot spot or visible neon glow; substantially uniform electron emission occurs over the entire area and a long operating life results.

The invention itself, together with further objects and advantages, may best be understood from the following description of preferred embodiments to be read in conjunction with the accompanying drawing. The features of the invention believed to be novel will be more particularly set forth in the appended claims.

In the drawing:

FIG. 1 illustrates diagrammatically a fluorescent lamp in which the invention may be embodied.

FIG. 2 illustrates pictorially a two-strand cathode embodying the invention.

FIG. 3 illustrates similarly a four-strand cathode.

FIG. 4 illustrates similarly a six-strand cathode arranged in a series parallel configuration.

Referring to FIG. 1, the low pressure fluorescent discharge lamp 1 comprises an elongated cylindrical envelope 2 into whose opposite ends are sealed a pair of electrodes 3, 3'. The electrodes are only diagrammatically represented in FIG. 1 and may correspond in structure to the illustrations of FIGS. 2 to 4. The lamp contains a quantity of mercury indicated by droplet 4 exceeding in amount the quantity vaporized during operation. The inert starting gas may consist of neon or argon or a mixture o argon and neon preferably at a total pressure less than 2.5 millimeters. By way of example, a suitable filling for raising the operating voltage in order to achieve high loading is neon at 2 millimeters of mercury pressure. A phosphor coating indicated at 5 on the inside of the envelope wall converts the ultraviolet radiation produced by the discharge through the mercury vapor into visible light.

Referring to FIG. 2, one electrode mount embodying the invention comprises a relatively short stem tube 7 having its outer end 8 flared for sealing peripherally into the tube end. The inner end of the stern tube is formed into a press 9 through which are sealed current inlead wires 11, 12. The inward projections of the inlead wires are formed into transverse arms 11a, 12a across which are looped a pair of filaments 13, 14 in such fashion as to extend towards the electrode at the other end of the lamp. By way of example, each loop may be formed by wrapping a finer overwind wire of 1.0 mil tungsten wire at 373 turns per inch around a composite mandrel formed by a 4.6 mil principal tungsten wire and an 8.0 mil molybdenum filler wire laid alongside each other. In the second coiling, the product of the first coiling is wrapped at 48.4 turns per inch around a mandrel consisting of a mil molybdenum wire after which the molybdenum wires are dissolved out by acid treatment. The finer 1.0 mil tungsten wire then forms a loose overwind on the 4.6 mil tungsten Wire which is itself coiled. Two 5.5 centimeter lengths of the filament are then welded in a loop across the transverse arms as illustrated. Previous to sealing the mounts into lamp envelopes, the filaments are dipped into a suspension of alkaline earth carbonates. After the mounts are sealed to the envelope, the electrodes are activated by heat decomposing the alkaline earth carbonates to oxides. The lamp is exhausted and the filling introduced in the usual Way through the exhaust tube 15 of one of the mounts which is then tipped oif.

Referring to FIG. 3, the illustrated mount 3b is similar to that which has been described except that 4 parallel filament loops 16 to 19 are bridged across the transverse arms 11a, 12a of the inlead wires; the length of each loop may be 4 centimeters.

Mount 3c illustrated in FIG. 4 comprises six filament loops 20 to arranged in parallel pairs connected in series. This is achieved by breaking up the transverse arms into sections 11b, 11c and 12b, 12c separated by insulating glass beads 26, 27. Thus loops 20 and 21, 22 and 23, and 24 and 25 form parallel pairs all connected in series across the inlead wires. The filament may be formed in the manner previously described with reference to FIG. 1 and then stretched about 2 to l; the loops are 4 centimeters in length.

Lamps of the four filament bridge electrode type illustrated in FIG. 3 have been operated at 1.5 amperes in test lamps using for the starting gas neon at 2 millimeters pressure, and have operated successfully in excess of 8400 hours. Other lamps using the six filament bridge electrode of FIG. 4, again with the same starting gas filling, have been operated at 1.5 and 3 amperes up to 10,000 hours. By comparison, control lamps using conventional electrodes which operate with a hot spot have only lasted under like conditions from 100 to 285 hours.

Observation of the bridge electrodes according to the invention indicate operation without hot spot with substantially equal division of current between the various filaments or branches. There is no visible neon glow and no observable sputtering. Although preheat current is required to provide emission for starting at the normal open circuit voltages, it may be discontinued during operation without producing hot spotting. The electrodes present a large area and no separate additional anode structures are required; the heating resulting from electron collection on the anode half cycle is utilized to maintain an optimum temperature.

Although the mode of operation of the extended bridge electrodes is not perfectly understood, it is believed that the following considerations account at least in part for the main features. Proceeding from the cathode, the various lasma regions encountered are:

(1) The sheath or cathode dark space which is a positive ion sheath substantially devoid of electrons and having a thickness from .1 to 1 millimeter, depending upon the starting gas pressure.

(2) The negative glow region wherein there are substantially equal concentrations of electrons and positive ions with substantially no gradient, but wherein the electrons have greater velocities than in any other region due to their acceleration through the positive ion sheath. The negative glow region may extend out as much as several centimeters, again depending upon the starting gas pressure.

(3) The Faraday dark space which is a region of negative gradient and wherein the substantially equal concentrations of electrons and positive ions drop gradually to the levels existing in the positive column. This region may extend from 1 to 5 centimeters beyond the negative glow region, depending upon the activity of the cathode; the more active the cathode, the shorter the Faraday dark space.

(4) The positive column which is a region of substantially equal concentrations of electrons and positive ions with a positive gradient extending the major length of the lamp towards the opposite electrode operating as anode.

The thickness of the positive ion sheath corresponds generally to the electron mean free path. When the sheath thickness is greater than the spacing of the secondary or larger turns in the coiling of the filament, the sheath will surround the coil as a whole and the ions coming in at the edge of the sheath will see the cathode as a cylinder and not as individual coil turns. Thus the effective collecting area of the cathode for ions is that of the sheath and not the geometrical area of the turns of the coil. Therefore, an extended or stretched out filament or cathode has a greater effective area and provides the required current with less current density at the edge of the sheath than in the case of a compact cathode. This means a lower ion :and electron density in the negative glow region and of course a lower cathode fall. With a lower cathode fall, there is of course less destructive bombardment of the cathode by the positive ions.

In the case of a hot spot, the current densitiesof ions and electrons just outside the spot may be enhanced as much as a hundred times. The tendency of the cathode to hot spot is believed caused by instability resulting from strong local field enhancement of electron emission due to statistical fluctuations in ion current density at the cathode. When the cathode is stretched out, the ion current density is less and therefore there is less tendency to formation of a hot spot and the higher cathode fall which it entails.

An extended cathode is better able to compete with the walls for ions. At some distance beyond the edge of the sheath, there is a regional potential maximum. With- 1n this maximum, ions are drawn by the field toward the cathode; beyond this maximum, ions are drawn to the walls. The lower the starting gas pressure, the further out this maximum will be and the smaller the fraction of the 1011s that can be collected by the cathode. Therefore in such case, by stretching out the cathode, it can collect a greater fraction of the ions created and this results in a lower cathode fall.

Considerations of probability of ionization and the geometry of a concentrated cathode as against an extended cathode also indicate that the reasons why stretching out the cathode lowers the cathode fall and improves life become more compelling the lower the starting gas fill pressure.

The specific embodiments of the invention which have been illustrated and described in detail are intended as exemplary only and the scope of the invention is to be determined by the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A low pressure electric discharge lamp comprising a vitreous envelope defining an elongated discharge channel having a pair of activated electrodes sealed therein at opposite ends and containing an ionizable medium comprising mercury and an inert starting gas at a pressure of a few millimeters of mercury, each electrode comprising a coiled tungsten filament extended in length such that the turn spacing in the coiling thereof is comparable to the turn diameter, at least 4 centimeters of such extended filament being provided per ampere of discharge capacity in each electrode in order to prevent formation of a cathode hot spot.

2. A lamp as defined in claim 1 wherein the inert starting gas is at a pressure less than 2.5 millimeters of mercury and contains a substantial proportion of neon.

3. A low pressure electric discharge lamp comprising a virteous envelope defining an elongated discharge channel having a pair of electrodes sealed therein at opposite ends and containing an ionizable medium comprising mercury and an inert starting gas at a pressure of a few millimeters of mercury, each electrode comprising a tungsten filament having a finer tungsten wire wound thereon in a loose overwind, said filament being itself coiled but extended in length such that the turn spacing in the coiling thereof is comparable to turn diameter, and an activating coating of alkaline earth oxides on said filament.

4. A lamp as defined in claim 3 wherein the length of extended coil filament in each electrode is at least 4 centimeters per ampere of discharge capacity.

5. A lamp as defined in claim 4 wherein the length of extended coiled filament in each electrode is disposed as several parallel strands.

6. A lamp as defined in claim 5 wherein the inert starting gas is at a pressure less than 2.5 millimeters of mercury and contains a substantial proportion of neon.

7. A lamp as defined in claim 3 wherein the tungsten filament consists of approximately 4.6 mil wire, the loose overwind thereon consists of approximately 1.0 mil wire, and the filament with over-wind is coiled in convolutions of approximately 10 mil diameter with the turn spacing roughly equal to the turn diameter, approximately 4 centimeters of such extended coiled filament being provided per ampere of discharge capacity in each electrode in order to prevent formation of a cathode hot spot.

References Cited UNITED STATES PATENTS 2,067,141 1/1937 Gustin 313211 X 2,094,668 10/1937 Pirgni et al 313212 X 2,153,008 4/1939 Scott 313-2l2 X 2,774,918 12/1956 Lemmers 315-98 JAMES W. LAWRENCE, Primary Examiner. R. F. HOSSFELD, Assistant Examiner.

U.S. Cl. X.R. 

